1. Technical Field
The present application relates generally to the detection of pollutants in air. In particular, the present application relates to a fiber optic gas sensor which can detect NO2 in a mixture of gases. The sensor is capable of quantitatively measuring nitrogen dioxide gas uniformly blended in air at concentrations as low as a few parts per billion.
2. Background of the Technology
Nitrogen oxides (NOx) are recognized as one of the primary air pollutants. The continued or frequent exposure may cause respiratory illness or harm the lung functions in human beings. Apart from the health hazards, NOx are important precursor to both ozone and acid rain. A major part of this pollutant gas is released from localized sources, primarily from the combustion factories and automobile exhausts. The continuous monitoring as well as quantitative detection of these gases at trace amount is always essential. A mechanism that provides accurate information about the pollutant level in the environment and enables to take the adequate steps to keep the normal environmental standards is highly needed.
Conventionally the measurement of NO2 concentrations has been carried out using analytical instruments based on the Saltzman method and chemiluminescence technique, as explained in Saltzman, “Colorimetric Microdetermination of Nitrogen Dioxide in the Atmosphere”, Anal. Chem. 26, 1949-1955 (1954); and Harper et al., “Detection of Nitric Oxide and Nitrogen Dioxide with Photoluminescent Porous Silcon”, Anal. Chem. 68, 3713-3717 (1996). Recently efforts to develop devices to quantitatively detect these highly toxic pollutants have come forward. The development of various devices based on semiconductor gas sensing, solid-electrolyte gas sensors, electrochemical gas sensors and quartz crystal gas monitors etc. are few examples in the gas sensing devices in this direction. These devices have certain drawbacks. For example, some of these devices require high operating temperatures. Others require a reference electrode or exhibit short lifetimes. In all these methods the sampling of the gas is a difficult procedure.
A common simple measurement method is to receive air using a pump, collect NO2 gas by directly collecting it into a sampling bag (i.e., a direct collection method). An alternative method involves using a solid-state adsorbent (i.e., a solid collection method) or by collecting into an absorbing solution (i.e., a liquid collection method) and analyzing the collected gas by the gas chromatography. In all of these methods it is necessary to transport not only samples but also peripheral apparatuses such as a pump etc. In the direct collection method, it is difficult to store the gas because the size of the sampling bag is limited. The solid collection method and the liquid collection method require a process of sensing the collected gas. In comparison to these conventional NO2 sensing schemes, fiber optic sensors (FOS) are particularly a promising alternative. For example, the sensor can be installed at different locations and eliminates many of the difficulties rendered by other sensing schemes.
Fiber optic sensors (FOS) have specific advantages such as geometrical versatility, small dimension, lightweight and remote sensing capabilities etc. Moreover these sensors offer complete electrical isolation and are totally isolated from the electromagnetic and radio frequency interferences. See, for example, Grattan and Meggitt. Eds., “Optical Fiber Sensor Technology: Fundamentals”, Kluwer Academic Publishers, Dordrecht (1999) and Krohn, “Fiber Optic Sensors: Fundamentals and Applications”, ISA-Instrumentation, Systems, and Automation Society, North Carolina (2000). Also a variety of applications of fiber optic sensors are disclosed in Culshaw, “Optical Fiber Sensing and Signal Processing”, Peter Peregrinus Ltd., London (986) and “Optical Fiber Sensors: Components and Subsystems”, Artech House, Inc., London (1996).
U.S. Pat. No. 6,362,005 B1 discloses a nitrogen dioxide gas sensing method based on the basic Saltzman method wherein a mixture of a diazotizing reagent which reacts with nitrous ions to produce a diazo compound and a coupling reagent that couples with a diazo compound to produce an azo dye are impregnated into a porous glass chip having fixed dimensions. An acid is also placed in the pores of this transparent body. The sensor element is placed in between a light emitting diode and a phototransistor followed by a voltmeter which measures the voltage difference. In this sensor, an easy leaching of the reagents from the pores of the sensor element is a serious problem. This limits the suitability of the sensor in different environmental conditions and also for long term use.
U.S. Pat. No. 5,567,622 describes a fiber optic chemical dosimeter system capable of detecting the presence of hydrazine fuels and nitrogen tetroxide and nitrogen dioxide gases that are used at rocket launch sites. The sensor is based on the detection of optical reflections from different sensor sites by the interrogation of infrared or visible red laser by an optical time domain reflectometry (OTDR) monitor.
There still exists a need for a durable and low cost device capable of detecting nitrogen oxide gases quantitatively and at low concentrations.